1. Technical Field to which the Invention Belongs
The present invention relates to a hydraulic dynamic bearing comprising a flanged shaft having a thrust ring portion and a cylinder portion, a single bag-like stepped sleeve in which there are formed in order a small diameter cylinder portion, a large diameter cylinder portion and a cylindrical open end portion, a presser ring pressure-inserted and fixed to the cylindrical open end portion of the sleeve, and a lubricating oil sealed in a fine gap including a thrust gap formed between these bearing constituting members, and to a spindle motor having this hydraulic dynamic bearing. In particular, it relates to a structure of a thrust dynamic pressure generating groove of the hydraulic dynamic bearing.
2. Prior Art
In Japanese Patent Laid-Open No. 32828/2001 Gazette, there are disclosed a hydraulic dynamic bearing comprising a flanged shaft having a thrust ring portion and a cylinder portion, a single bag-like stepped sleeve in which there are formed in order a small diameter cylinder portion, a large diameter cylinder portion and a cylindrical open end portion, a presser ring pressure-inserted and fixed to the cylindrical open end portion of the sleeve, and a lubricating oil sealed in a fine gap including a thrust gap formed between these bearing constituting members, characterized in that a thrust dynamic pressure generating groove provided either of a stationary face and a rotation face, which form the thrust gap, is formed such that its depth becomes gradually deep from a side at which a flow velocity of the lubricating oil flowing through the thrust gap is low toward a side at which it is high, and a spindle motor having this hydraulic dynamic bearing.
In a thrust dynamic bearing portion adopting the thrust dynamic pressure generating groove of such a structure, the lubricating oil that is a pressure generating fluid held between the shaft and the sleeve is pulled into the thrust dynamic pressure generating groove by a relative rotary motion of both the bearing constituting members. A pattern of this thrust dynamic pressure generating groove is an optional pattern such as spiral and herringbone.
Although the lubricating oil flows in the thrust dynamic pressure generating groove by a flow line complying with the pattern, since a depth of the thrust dynamic pressure generating groove becomes shallow as going from an inlet to an interior along the flow line, a dynamic pressure in the thrust gap becomes high as a whole. In short, the hydraulic dynamic bearing adopting the thrust dynamic pressure generating groove of the structure mentioned above can efficiently generate a higher thrust dynamic pressure in comparison with the hydraulic dynamic bearing in which the groove depth does not change along the flow line. A pressure pattern of the thrust dynamic pressure of this thrust dynamic bearing portion is as shown in FIG. 5.
However, as shown in FIG. 5, in the thrust dynamic bearing portion of the structure mentioned above, since the lubricating oil is abruptly pulled into the thrust dynamic pressure generating groove, the lubricating oil is subjected to a negative pressure in the vicinity of an inlet, so that bubbles are generated in that place. Whereupon, such a situation occurs that the lubricating oil flowing through the thrust gap at high speed is interrupted, so that there arises such an unfavorable situation that the thrust dynamic pressure becomes unstable and thus a deflection of an NRRO, etc. is increased. And, in the worst case, the shaft is seized by the sleeve, and thus the rotation is stopped.
Further, even if it is the thrust dynamic pressure generating groove formed with the groove depth being kept constant, a pressure in its flange diameter direction becomes a negative pressure. That is, if the maximum pressure and the flange diameter direction pressure are simulated by changing the depth of the thrust dynamic pressure generating groove in a range of 5 μm to 20 μm, the maximum pressure changes in a range of 100 kPa to 140 kPa and the flange diameter direction pressure changes in a range of−30 kPa to−40 kPa as shown in FIG. 7. In either groove depth, the flange diameter direction pressure is a negative pressure.
In case where the thrust dynamic pressure generating groove of such a spiral pattern that the groove depth becomes gradually deep from the side at which the flow velocity of the lubricating oil flowing through the thrust gap is low toward the side at which it is high, is provided in upper and lower faces of a thrust ring 3 of a flanged shaft 1 constituting the hydraulic dynamic bearing of a spindle motor of FIG. 1, the negative pressure mentioned above is generated in a fine gap between an outer peripheral face of the thrust ring 3 and an inner peripheral face of a large diameter cylinder portion of a stepped sleeve 4. Further, in case where the thrust dynamic pressure generating groove of a herringbone pattern is provided, the negative pressure mentioned above is generated in the fine gap between the outer peripheral face of the thrust ring 3 and the inner peripheral face of the large diameter cylinder portion of the stepped sleeve 4, and in the vicinities of shaft sides in the upper and lower faces of the thrust ring 3.
Further, also in case where the thrust dynamic pressure Generation groove whose depth is kept constant is provided in Upper and lower faces of the thrust ring 3 of the flanged shaft 1 constituting the hydraulic dynamic bearing of the spindle Motor of FIG. 1, thew negative pressure is generated similarly to the above